The present invention relates to a voltage measuring apparatus which measures voltage of a voltage source, and more particularly, to a technique for enhancing measurement precision.
In terms of environmental protection, there are known electric cars having a motor as a driving source for driving the car, and hybrid vehicles having a motor and an engine. As an electric power source for driving the motor, several hundred storage batteries or fuel cells (generally called “batteries”, hereinafter) are connected to one another in series and used in many cases. Therefore, in order to monitor a state-of-charge, a state-of-discharge, life of each battery, abnormal condition and the like which are varied from moment to moment during running, there is an increasing requirement for precisely measuring terminal voltage of battery cells which are connected to one another in series and to which a few hundred volts of electrical power is applied.
To meet such a requirement, measurement of terminal voltage of a battery is carried out in such a manner that a plurality of battery cells constituting a battery are modularized, a capacitor element is recharged using voltage of each module through a first switch group, the first switch group is turned OFF and then, the capacitor element is connected to a voltage measuring device through a second switch group.
For example, in Japanese Patent Application Laid-open No. 2001-116777, a photo MOSFET or an element corresponding to the photo MOSFET is used as a switch element. It is easy to separate a switch driving signal system and a measuring voltage signal system from each other, a switch driving signal is not easily affected by measuring voltage, and no special arrangement is required for a switch driving circuit.